Herpes Simplex is an enveloped double-stranded DNA virus that is responsible for primary and recurrent infections in humans and is related to the viruses that cause infectious mononucleosis (Epstein-Barr Virus), chicken pox and shingles (Varicella Zoster Virus). Symptoms of Herpes Simplex Virus (HSV) infections include an eruption of tiny blisters on the skin or mucous membranes. After the eruption of blisters subsides, the virus remains in a dormant (latent) state inside the group of nerve cells (ganglia) that supply the nerve fibers to the infected area. Periodically, the virus reactivates, begins growing again, and travels through the nerve fibers back to the skin, thereby causing eruptions of blisters in the same area of skin as the earlier infection. Sometimes the virus may be present on the skin or mucous membranes even when there is no obvious blister. Herpes Simplex Virus (HSV) is classified into two types, HSV-1 and HSV-2. The complete genomes of human HSV-1 and HSV-2 have been sequenced (see, for example, NCBI Accession Nos. X14112 and Z86099, respectively).
HSV has been shown to contribute to or cause a variety of disorders, including blindness and encephalitis. Besides causing local outbreaks, HSV-1 and HSV-2 are associated with encephalitis. The pathophysiology of this encephalitis is poorly understood in humans. Animal models suggest that the virus enters the central nervous system through peripheral nerves and causes inflammation of the brain. HSV-1 is the more common cause of adult encephalitis. HSV-2 is the more common cause of newborn encephalitis, which is associated with maternal genital infections. HSV-2 is one of the most common sexually transmitted diseases in society. HSV-related encephalitis has the highest fatality rate of all the types of encephalitis with an annual incidence of 1 to 4 per million. HSV encephalitis affects people of all ages and at any time of the year. In adults, HSV-related encephalitis is thought to be due to a reactivation of a latent virus. Symptoms may include fever, headaches, seizures, an altered level of consciousness and personality changes. The similarity of these symptoms to other maladies makes clinical diagnosis difficult. If left untreated, the mortality rate for herpes simplex encephalitis (HSE) is as high as seventy percent, compared with as low as nineteen percent among those who receive treatment. Of the treated patients, approximately thirty-eight percent are reported to eventually return to normal function. It is, therefore, very important to be able to diagnose HSV infection at an early stage.
The diagnosis of HSV infection is commonly performed using cell culture on appropriate clinical specimens. However, the ability to isolate HSV in cell culture is reduced in old lesions, in the presence of a host immune response and in episodes of reactivation. Serologic diagnosis, particularly of HSV in cerebrospinal fluid (CSF), is not sufficiently sensitive or specific, and takes too much time to be of use in decisions involving choices for early therapeutic intervention of encephalitis. HSV is rarely detected in cerebral spinal fluid using cell culture, with only four percent of the cases being culture-positive. Serological methods are also inadequate for diagnosis of HSE due to delay between two and three weeks in appearance of antibody response after initial infection. The “gold standard” method of diagnosis involving brain biopsies is invasive and controversial with significant risk of long-term morbidity. Alternate techniques such as Computer-Assisted Tomography and Magnetic Resonance Imaging are not specific and lack sensitivity as diagnostic tools.
At the present time, immunological methods for detection of HSV are unreliable and difficult to perform. Molecular methods of detection offer the potential for enhanced sensitivity and faster time to result than is possible by conventional means. There are instances in which rapid, sensitive, and specific diagnosis of HSV disease is imperative. There is therefore, a clinical need to develop a rapid and sensitive tool to aid in the diagnosis of HSV. There also remains a need for a tool for the typing of the HSV infection. Rapid identification of the specific etiological agent involved in a viral infection provides information which can be used to determine appropriate therapy within a short period of time.